Special Relativity Clocks

JM

So, what is your definition of proper time?

OK, so where do I find out about these things?
JM

JM

I know this is an early post, but it seems relavent now.

Clocks do only what they are told. The theory says that the time of the moving frame is given by c t' = gamma(c t - v x / c ). This means that the moving clock is instructed ( or built ) to accept t, x, and v/c as inputs and to display the result as t'. Thus the moving clock has no initiative of its own to decide what time to display, but must display what the stationary frame tells it to.
JM

DaleSpam

Clocks measure proper time. If something doesn't measure proper time then it isn't a clock.

Proper time doesn't need any reference to a mythical "stationary" frame.

JM

Refer to 1905 section 4: "It is at once apparent that this result still holds good if the clock moves from A to B in any polygonal line, and also when the points A and B coincide."
It is not apparent to me. If it is to you,can you explain it to me?
Where are the points A and B in terms of x,y,z,t, and where is the polygonal line? The theory of section 3 refers to clocks moving parallel to x, so how to make a polygon out of that? The picture that sentence suggests to me is a series of stationary frames, each one aligned along one segment of the polygonal line, with an accompanying moving frame. The change of direction from one segment to another implies an acceleration of the clock. I don't see anything in section 3 about that. If one clock is on the equator and the other is at the pole then their positions will never coincide. So what is the explanation?
See the posts on page one of this thread, and the one just above.
The confusion was about the meaning of the phrase 'moving clocks run slow'. It was cleared up with the qualifier that proper clocks run slow, not arbitrary coordinate clocks. We didn't get to what a correct definition of a proper clock is. I am fine with inertial clocks being slow, but not non-inertial ones. As I noted above, I dont see how the inertial analysis of section 3 applies to non-inertial clocks.
I have that book, and I have read it. I dont recall anything about clocks moving in various directions, or all clocks being proper. I was hoping that you could suggest a text better than Taylor.
Thanks again for your efforts.
JM

ghwellsjr

In section 3, the clocks were moving parallel to x because it is conventional in the standard configuration of the Lorentz Transformation to align the axes so that the motion is along the x-axis. It doesn't matter physically which direction the motion is in, we just assign the two co-ordinate systems so that the relative motion between them is along the x-axis. Remember, all frames are equally valid, including ones where the only difference is the orientation of their axes.

So once Einstein establishes that any clock that moves in a reference frame along the x-axis will tick at a slower rate than the co-ordinate clocks of that reference frame, he generalizes the concept to include any clock moving in any direction and he says to pick any two additional clocks, one at any point A and one at any other point B, not necessarily aligned along the x-axis, which had previously been synchronized with each other when at relative rest, and move the one at A to the position of the one at B at some relatively slow velocity v, then when it gets there, it will be slow by ½tv²/c² compared to the clock at B. (Note that this formula is approximate and only applies to a slow-moving clock.)

Then he says that we can repeat the process, moving the A clock from the first B position to another B position in any other direction and we will get the same additional difference in clock time when it gets there. We can repeat the process as many times and in as many directions as we want, even to the point where we eventually return the A clock to its original location and the same formula applies if we take the total time t for the clock to make its round trip. This is what he means by the A and B points coinciding.
You can do the analysis with multiple additional frames if you want, but it is just more complicated with no additional increase in knowledge.
Prior to space travel (or sustained air travel), this was the only way to carry out the experiment. And it still would work, neglecting any effects from gravity, even if the clocks don't ever come to the same location because we are considering just one inertial rest frame, that of the clock at the pole. But of course nowadays, we just have the moving clock take off in a space ship (or airplane, which has been done).

Don't be confused by the oft-repeated statement that clocks have to be co-located at the start and end of the journey of one of them to compare times. All frames will show that there is a difference in accumulated times, even if they don't agree on the absolute times on the two clocks (because of simultaneity issues).
You repeated several times that man-made clocks do what we tell them to do but let's assume that they all have one thing in common, they tick once per second. Then the only issue is how many ticks have transpired between point A and point B, agreed? In this sense, we can treat them as stop watches, even if they actually display time as hours, minutes, and seconds or if they count backwards.

But the point is that no one makes a clock that is designed to tick slowly when it is traveling with respect to some rest frame--how in the world would they do that? And you overlook that fact that two identical clocks in inertial relative motion would each tick more slowly compared to its own tick rate. How do you design clocks to do that? No, it happens independently of any purposeful design, in fact if you tried to make it happen, it wouldn't be reciprocal.
It wasn't cleared up by the qualifier that only "proper clocks" run slow and we did get to the correct definition of a "proper clock". But it is not a generally acknowledged definition. It is what I would call a private definition made by Taylor and Wheeler on page 10 of their book which you pointed out. No one else talks about a "proper clock". Instead, we keep repeating that all clocks keep track of "proper time". This applies to inertial clocks and non-inertial clocks, moving clocks, stationary clocks, accelerating clocks and co-ordinate clocks. All clocks keep track of their own proper time. They don't have any choice. Of course we are talking about carefully designed clocks that aren't affected by environmental effects, such as a pendulum clock.
Well, I hope you can see it now.
Here is the link to Einstein's 1920 book: http://www.bartleby.com/173/.

Now if you look at the end of chapter 12, you will see this statement:
Then if you look at chapter 23, you will see where Einstein once again discusses a clock moving in a circle with respect to a stationary clock.

DaleSpam

All moving clocks run slow, not just proper clocks. See the formula I posted above. It applies to all clocks, inertial or non inertial.

What is a coordinate clock? That is also a non standard term. Is it defined somewhere or are you just making things up?

Mentz114

The accepted definition of proper time is the Lorentzian ( or proper) length of a segment of a worldline. Consider a piece of string. The length of the string is independent of its shape because we use the Euclidean definition ,
Length = √( x²+y²+z²)

But worldlines are 4-dimensional and the proper length is given by the Lorentzian length,
L = √( c²t²-x²-y²-z²) or T = √( t²-x²/c²-y²/c²-z²/c²)

with this definition of length, the strings length depends on its shape. A twisty bent piece of string is shorter than it would be if measured stretched out.

This is why the travelling twin is younger than the stay at home twin

Read, ask and listen. You could start here

http://en.wikipedia.org/wiki/Proper_time

JM

OK.
Suppose that A is at (x,y,z) = (0,0,0) and B is at (x,y,z) = ( 1,1,0). Within section 3, where all clocks move parallel to x, there is no clock that passes through these two points. One option is to add an extra clock that is not at rest in either K', moving or in K, stationary. But would that clock follow the same 'slow clock' formula? Another option is to align the axes of the moving frame with the line passing through these two points. For this option the moving frame must then be re-aligned again to get the clock to the second point B. This is the option that I refer to. Do you see another option?

In general,OK. But lets examine the idea that they all tick once per second. The clocks of K, stationary, can be considered as reference clocks and ,for convenience, assumed to be adjusted to match the day-night cycle of the earth( on average). This is what I call everyday time. But what about the moving clocks? Are you saying that the moving clocks also are adjusted to everyday time? i.e. that one second on a moving clock is the same as one second on a statioinary clock? The transform equations seem to demand this; if t is entered in seconds then the resulting t' must be also measured in the same seconds. Also, section 3 asserts that ' the clocks are in all respects alike'. But if this is so, then all clocks are running at the same rate, so how can a moving clock be said to run slow? Could it be a matter of terminology (or translation), with the meaning actually being that an interval between two events is different as measured by moving clocks than by stationary clocks, even thouth both clocks are running at the same rate? This seems to me to be a better view, because when comparing two things it is necessary to use the same units, if the clocks are running at different rates then the measurements are meaningless.

Can I infer from this that your definition of proper time is simply the time of any clock? With no need for any 'events' for the clock to mark?

I have checked and my book has the same statements in the same places, so we're talking about the same book. Aren't the items you refer to just re-statements of the same things as presented in 1905?

I am still puzzled by your, and DaleSpams, reluctance to identify published sources for your ideas. Surely there must be some, what gives? The responses to my posts suggest that there is a line of theory that is not wholly included in Einsteins works. I have heard of world lines, maybe in French, and Taylor hints at a different viewpoint. Us old timers prefer paper books to internet, maybe because of editing and reviewing.

I can see that you have put much effort into this conversation, and I appreciate it.
JM

JM

Don't these definitions mean that a line between two points connected by a light ray has zero length? Does that make sense?

[/QUOTE]http://en.wikipedia.org/wiki/Proper_time[/QUOTE]
Thanks for the reference, I'll look into it.
JM

DaleSpam

I don't understand this comment at all. I posted the Wikipedia link on proper time back in post 30-something when I first joined this thread. Please start there, it will be the best resource for an introduction, and is entirely sufficient for this conversation.

Once you understand the Wikipedia page then you can search for "spacetime interval" or "line element" or "spacetime metric" or "Riemannian metric" for more information, but most of that will be too advanced until you have mastered the material on the Wikipedia page.

If you specifically want paper-published sources then any introductory SR textbook will include material on proper time although it may be called "spacetime interval", or "invariant interval". You have some textbooks already, just start in the index there if you don't like Wikipedia.

DaleSpam

Yes. Such lines are called "null" or "lightlike".

ghwellsjr

Since Einstein was living in an era where fast space travel was not possible, he was merely simplifying the analysis by using the 'slow clock' formula but the exact formula will still work and should be used where the difference between the two answers would be significant. So this option is perfectly viable.
This is the option I put in bold above.
I don't see the need for another option, do you?
There was a time, many decades ago, when the rotation of the earth was the most stable standard for a second, but now that we can make atomic clocks that are more stable, it would be meaningless to continue with that standard and so now we use atomic clocks as a standard to define what a second means. But that presents the problem that you are asking about. Not only will moving clocks tick at different rates (as analyzed by Special Relativity) but clocks at different altitudes will also (as analyzed by General Relativity). So it is a real problem that has to be dealt with and fortunately we have very smart people who have come up with a solution to provide us with a coordinated everyday time which is called "Coordinated Universal Time". The clocks on board GPS satellites are examples of moving clocks that have to be adjusted to everyday time and our GPS devices take care of the problem so that we can all make it to our meetings at the same agreed upon time. But if you were doing physics experiments, such as measuring the speed of light, you would not use the time standard provided by GPS because you will get the wrong answer. Instead, you have to use your own atomic clock to provide you with the measurement of time.

We can't ignore the issue of moving clocks ticking at different rates and rather than saying it is all meaningless, we have agreed upon conventions to make the best sense out of the situation.
Yes, except it's not my definition, it was promoted by Minkowski in 1908. Einstein apparently didn't see the need to have a special term for something that applies to all clocks. But in terms of talking about the time between two arbitrary events, there is no single answer to that question because two clocks traveling in different ways between those two events can have a different answer. The term "proper clock" was coined to apply to an inertial clock that travels unaccelerated between those two events.
Yes, that was my point.
I think you are referring to Minkowski's re-interpretation and re-statement of Einstein's ideas. Einstein gave passing mention of his work in chapter 17 and near the end of his 1920 book. It is basically a graphical presentation of the Lorentz Transform and served as an important graphical aid in an era in which calculators and computers and videos were not available. But it is of necessity limited to one dimension of space. Nowadays, two-dimensional animated presentations are readily available to communicate the same ideas much more effectively. I never bothered to study Minkowski's work so I don't know what would be a good reference but I'm sure there are plenty.

ghwellsjr

When Mentz says "length", he means "spacetime interval" which can physically be either a spatial length or a time period, depending on the two events.

If an inertial clock can be present at the two events, then the spacetime interval is "timelike" and is the accumulated time on the clock. This is Taylor and Wheeler's definition of a "proper clock". There is an inertial frame in which the clock is at rest.

If the two events are so far apart that a physical clock could not get from the first event to the second event, then the spacetime interval is "spacelike" and is measured with an inertial ruler spanning between the two events and in a frame in which it is at rest. Taylor and Wheeler did not call this a "proper ruler" but they could have.

Since light rays don't have rest frames, the concept of a spacetime interval is meaningless for events that a light ray is present at both of them.

yuiop

I wouldn't agree that a null spacetime interval is entirely meaningless. It lies on the boundary between spacelike and timelike intervals, is called a "lightlike" interval and the events can be causally connected and is very useful for a lot of calculations.

JM

I looked at that Wike page and it looked like just a list of formulas, with no supporting theoretical foundation. Where is the foundation?
Yes, I have textbooks, but they don't answer the questions that I've asked in this thread, if they did I wouldn't have asked.
So, can you recommend a specific ' introductory SR textbook' , or not?
JM

JM

Well, George, we seem to be out of synch again.
I still don't see the principles or math that justify the generalization from clocks moving along x to clocks moving in arbitrary directions.
At one point you seemed to say that all clocks (did you mean both moving and stationary?)tick at the same rate, ie one tick per second. I cited reasons to believe this. Then, above, you say that moving clocks tick at a different rate. So, which is it?
So you don't read Minkowski, don't like Taylor, and don't have a suggested text. Then where do you get your ideas about SR?
JM

DaleSpam

Wikipedia always puts a list of references and external links down at the bottom. In this case, the theoretical foundation is pretty simple so there are only a couple of references. The rest is a more practical introduction, which I found very helpful.

I cannot recommend an introductory SR textbook, mine was terrible and I found Wikipedia much better. I would recommend starting with chapter 1 of Sean Carroll's lecture notes on GR:
http://arxiv.org/abs/gr-qc/9712019

Chapter 1 is just SR, and he introduces the spacetime interval on page 3 and makes the connection to proper time on page 26.